1. Field of the Invention
The present invention relates to a spin-valve type magnetoresistive sensor wherein electrical resistance is changed depending on the relationship between a stationary magnetization direction of a pinned magnetic layer and a magnetization direction of a free magnetic layer which is affected by an external magnetic field. More particularly, the present invention relates to a spin-valve type magnetoresistive sensor having superior heat resistance, a magnetoresistive head incorporating the spin-valve type magnetoresistive sensor, and to a method of manufacturing the spin-valve type magnetoresistive sensor, by which the magnetization direction of the free magnetic layer and the magnetization direction of the pinned magnetic layer can be easily set in orthogonal relation.
2. Description of the Related Art
There are known two types of magnetic heads utilizing a magnetoresistive effect; i.e., an AMR (Anisotropic Magnetoresistive) head incorporating a sensor which exhibits a magnetoresistive effect and a GMR (Giant Magnetoresistive) head incorporating a sensor which exhibits a giant magnetoresistive effect. In the AMR head, the sensor exhibiting a magnetoresistive effect has a single-layer structure formed of a magnetic substance. On the other hand, the GMR head comprises a multilayer structure sensor made of a plurality of materials formed as a laminate of layers. There are several types of structures capable of developing a giant magnetoresistive effect. A spin-valve type magnetoresistive sensor is known as having a relatively simple structure and providing a high rate of resistance change with respect to a weak external magnetic field.
FIGS. 13 and 14 are sectional views each showing the structure of an example of conventional spin-valve type magnetoresistive sensors, as viewed from the side facing a recording medium.
Above and below the spin-valve type magnetoresistive sensor of each example, shielding layers are formed with gap layers interposed therebetween. The spin-valve type magnetoresistive sensor, the gap layers, and the shielding layers cooperatively construct a GMR head for reproduction. An inductive head for magnetic recording may be layered on the GMR head for reproduction. The GMR head for reproduction is provided, for example, on a trailing end face of a floating slider along with the inductive head for magnetic recording, whereby a magnetoresistive head is constructed. The magnetoresistive head is used to detect a magnetic field recorded on a magnetic recording medium such as a hard disk.
In FIGS. 13 and 14, a Z-direction represents the moving direction of a magnetic recording medium, and a Y-direction represents the direction of a leakage magnetic field from the magnetic recording medium.
The spin-valve type magnetoresistive sensor shown in FIG. 13 is one example of the so-called bottom type single-spin-valve magnetoresistive sensors wherein an antiferromagnetic layer, a pinned magnetic layer, a non-magnetic electrically conductive layer, and a free magnetic layer are formed on a substrate one by one in this order from the substrate side.
The spin-valve type magnetoresistive sensor shown in FIG. 13 comprises a multilayer film 33 made up of an underlying layer 31, an antiferromagnetic layer 22, a pinned magnetic layer 23, a non-magnetic electrically conductive layer 24, a free magnetic layer 25 and a protective layer 32, which are formed in this order from the lower side in FIG. 13; a pair of hard bias layers (permanent magnetic layers) 29, 29 formed on both sides of the multilayer film 33; and a pair of electrode layers 28, 28 formed respectively on the hard bias layers 29, 29.
The underlying layer 31 and the protective layer 32 are each formed of a Ta film or the like. Also, a track width Tw is determined by the width of an upper surface of the multilayer film 33.
In general, the antiferromagnetic layer 22 is formed of a Fe—Mn alloy film or a Ni—Mn alloy film, and the pinned magnetic layer 23 and the free magnetic layer 25 are each formed of a Ni—Fe alloy film. The non-magnetic electrically conductive layer 24 is formed of a Cu film, the hard bias layers 29, 29 are each formed of a Co—Pt alloy film, and the electrode layers 28, 28 are each formed of a Cr or W film.
As shown in FIG. 13, magnetization of the pinned magnetic layer 23 is brought into a single domain state in the Y-direction (the direction of a leakage magnetic field from the magnetic recording medium; the direction of height) under an exchange anisotropic magnetic field cooperatively generated in the antiferromagnetic layer 22. Magnetization of the free magnetic layer 25 is uniformly arranged in a direction opposing to the X1-direction under the effect of a bias magnetic field from the hard bias layers 29, 29.
In other words, the magnetization of the pinned magnetic layer 23 and the magnetization of the free magnetic layer 25 are set to cross in orthogonal relation.
In the spin-valve type magnetoresistive sensor shown in FIG. 13, a detection electric current (sensing electric current) is applied from the electrode layers 28, 28 formed on the hard bias layers 29, 29 to the pinned magnetic layer 23, the nonmagnetic electrically conductive layer 24 and the free magnetic layer 25. The magnetic recording medium such as a hard disk travels in the Z-direction. When a leakage magnetic field from the magnetic recording medium is applied in the Y-direction, the magnetization of the free magnetic layer 25 is varied from the direction opposing to the X1-direction toward the Y-direction. Electrical resistance is changed (called a magnetoresistance change) depending on the relationship between a variation of the magnetization direction in the free magnetic layer 25 and the stationary magnetization direction of the pinned magnetic layer 23. The leakage magnetic field from the magnetic recording medium can be detected in accordance with a voltage change caused by such a change in electrical resistance value.
The spin-valve type magnetoresistive sensor shown in FIG. 14 is another example of the so-called bottom type single-spin-valve magnetoresistive sensors wherein an antiferromagnetic layer, a pinned magnetic layer, a non-magnetic electrically conductive layer, and a free magnetic layer are formed on a substrate one by one in this order from the substrate side (the lower side in FIG. 14).
In FIG. 14, character K denotes a substrate. An antiferromagnetic layer 22 is formed on the substrate K. Further, a pinned magnetic layer 23 is formed on the antiferromagnetic layer 22, and a non-magnetic electrically conductive layer 24 is formed on the pinned magnetic layer 23. Moreover, a free magnetic layer 25 is formed on the non-magnetic electrically conductive layer 24.
On the free magnetic layer 25, a pair of bias layers 26, 26 are formed while a spacing corresponding to a track width Tw is left between the bias layers 26, 26. A pair of electrically conductive layers 28, 28 are formed respectively on the bias layers 26, 26.
The pinned magnetic layer 23 is formed of, for example, a Co film, a NiFe alloy, a CoNiFe alloy, or a CoFe alloy. The antiferromagnetic layer 22 is formed of a NiMn alloy.
The bias layer 26 is formed of an antiferromagnetic material, such as a FeMn alloy, which belongs to the face-centered cubic system, has an irregular crystal structure, and does not require heat treatment for generating an exchange anisotropic magnetic field.
The pinned magnetic layer 23 shown in FIG. 14 is magnetized in one direction under an exchange anisotropic magnetic field based on exchange coupling produced at the interface between the pinned magnetic layer 23 and the antiferromagnetic layer 22. The magnetization direction of the pinned magnetic layer 23 is made stationary in the Y-direction shown in FIG. 14, i.e., the direction away from the magnetic recording medium (direction of height).
Also, the free magnetic layer 25 is magnetized into a single domain state under an exchange anisotropic magnetic field cooperatively generated in the bias layers 26. Then, the magnetization direction of the free magnetic layer 25 is uniformly arranged in a direction opposing to the X1-direction shown in FIG. 14, i.e., a direction perpendicularly crossing the magnetization direction of the pinned magnetic layer 23.
Because the free magnetic layer 25 is magnetized into a single domain state under the exchange anisotropic magnetic field cooperatively generated in the bias layers 26, the occurrence of Barkhausen noise is prevented.
In the conventional spin-valve type magnetoresistive sensor shown in FIG. 14, a steady electric current is applied from the electrically conductive layer 28 to the free magnetic layer 25, the non-magnetic electrically conductive layer 24 and the pinned magnetic layer 23. When a leakage magnetic field from the magnetic recording medium traveling in the Z-direction is applied in the Y-direction in the above condition, the magnetization of the free magnetic layer 25 is varied from the direction opposing to the X1-direction toward the Y-direction. Electrical resistance is changed depending on the relationship between a variation of the magnetization direction in the free magnetic layer 25 and the stationary magnetization direction of the pinned magnetic layer 23. The leakage magnetic field from the magnetic recording medium can be detected in accordance with a voltage change caused by such a change in electrical resistance.
The spin-valve type magnetoresistive sensor shown in FIG. 14 is manufactured as follows. As shown in FIG. 15, all the component layers from the antiferromagnetic layer 22 to the free magnetic layer 25 are successively formed on the substrate K one above another, and are then subjected to heat treatment (annealing) under a magnetic field. An exchange anisotropic magnetic field is thereby generated at the interface between the pinned magnetic layer 23 and the antiferromagnetic layer 22 to make the magnetization direction of the pinned magnetic layer 23 stationary in the Y-direction shown in FIG. 14. Further, as shown in FIG. 16, a lift-off resist 351 having a width substantially corresponding to the track width is formed. Then, as shown in FIG. 17, the bias layer 26 and the electrically conductive layer 28 are successively formed on a surface area of the free magnetic layer 25 which is not covered by the lift-off resist 351. After removing the lift-off resist 351, the magnetization direction of the free magnetic layer 25 is uniformly arranged in the direction of the track width. As a result, the spin-valve type magnetoresistive sensor having the magnetization direction shown in FIG. 14 is manufactured.
Next, FIG. 18 is a sectional view showing the structure of a principal part of a magnetoresistive head including still another example of conventional spin-valve type magnetoresistive sensors, as viewed from the side facing a recording medium.
In FIG. 18, symbol MR3 denotes a spin-valve type magnetoresistive sensor, and symbol al2 denotes a laminate. The laminate a12 is formed such that an antiferromagnetic layer 122 is formed on an underlying layer 121; a pinned magnetic layer is formed on the antiferromagnetic layer 122; a non-magnetic electrically conductive layer 124 is formed on the pinned magnetic layer 153; a free magnetic layer 175 is formed on the non-magnetic electrically conductive layer 124; and a protective layer 127 is formed on the free magnetic layer 175.
The free magnetic layer 175 in the spin-valve type magnetoresistive sensor MR3 of this example is made of a non-magnetic intermediate layer 176, a first free magnetic layer 177, and a second free magnetic layer 178, the first and second layers 177, 178 sandwiching the non-magnetic intermediate layer 176 therebetween.
The first free magnetic layer 177 is positioned closer to the protective layer 127 than the non-magnetic intermediate layer 176, and the second free magnetic layer 178 is positioned closer to the non-magnetic electrically conductive layer 124 than the non-magnetic intermediate layer 176. Further, the second free magnetic layer 178 is made up of a diffusion preventing layer 179 and a ferromagnetic layer 180.
The second free magnetic layer 178 has a thickness t2 greater than a thickness t1 of the first free magnetic layer 177. Also, assuming that saturation magnetization of the first free magnetic layer 177 and the second free magnetic layer 178 is respectively M1, M2, a magnetic film thickness of the first free magnetic layer 177 and the second free magnetic layer 178 is respectively M1·t1, M2·t2. Since the second free magnetic layer 178 is made up of the diffusion preventing layer 179 and the ferromagnetic layer 180, the magnetic film thickness M2·t2 of the second free magnetic layer 178 is given as the sum of a magnetic film thickness of the diffusion preventing layer 179 and a magnetic film thickness of the ferromagnetic layer 180.
Further, the free magnetic layer 175 is formed such that the magnetic film thicknesses of the first free magnetic layer 177 and the second free magnetic layer 178 satisfy a relationship of M2>t2>M1·t1, Moreover, the first free magnetic layer 177 and the second free magnetic layer 178 are coupled to each other in antiferromagnetic relation. In other words, when the magnetization direction of the second free magnetic layer 178 is uniformly arranged in the X1-direction shown in FIG. 18, the magnetization direction of the first free magnetic layer 177 is uniformly arranged in a direction opposing to the X1-direction.
In addition, since the magnetic film thicknesses of the first and second free magnetic layers 177, 178 satisfy the relationship of M2·t2>M1·t1, the magnetization of the second free magnetic layer 178 remains eventually and the magnetization direction of the free magnetic layer 175 is uniformly arranged as a whole in the X1-direction. At this time, an effective film thickness of the free magnetic layer 175 is given by (M2·t2−M1−M1·t1).
Thus, the first free magnetic layer 177 and the second free magnetic layer 178 are coupled to each other in antiferromagnetic relation so as to have antiparallel magnetization directions, and their magnetic film thicknesses satisfy the relationship of M2·t2>M1>M1·t1, whereby the free magnetic layer 175 is brought into an artificial ferrimagnetic state. Also, the magnetization direction of the free magnetic layer 175 and the magnetization direction of the pinned magnetic layer 153 cross each other.
The conventional spin-valve type magnetoresistive sensor shown in FIG. 13 however has a risk of causing the problem described below.
As mentioned above, the magnetization of the pinned magnetic layer 23 shown in FIG. 13 is brought into a single domain state in the Y-direction and held stationary, but the hard bias layers 29, 29 magnetized in the direction opposing to the X1-direction are provided on both sides of the pinned magnetic layer 23. Therefore, opposite ends of the pinned magnetic layer 23 are so affected by the bias magnetic field from the hard bias layers 29, 29 that it is difficult to hold the magnetization direction of the pinned magnetic layer 23 stationary in the Y-direction shown in FIG. 13.
More specifically, under the effect of magnetization of the hard bias layers 29, 29 in the direction opposing to the X1-direction, the magnetization of the free magnetic layer 25, which is brought into a single domain state in the direction opposing to the X1-direction, and the magnetization of the pinned magnetic layer 23 are hard to cross in orthogonal relation, especially, in the vicinity of the lateral ends of the multilayer film 33. The reason why the magnetization of the free magnetic layer 25 and the magnetization of the pinned magnetic layer 23 are held in orthogonal relation, resides in that the magnetization of the free magnetic layer 25 can be easily varied even with a small external magnetic field and electrical resistance can be greatly changed in such a condition, thus resulting in an improvement of reproduction sensitivity. Further, with the orthogonal relation between both the magnetization directions, an output signal waveform having better symmetry can be obtained.
Additionally, the magnetization of the free magnetic layer 25 in the vicinity of lateral ends thereof tends to be undesirably held stationary because of a strong effect of the magnetization of the hard bias layers 29, 29, and hence tends to vary less sensitively upon application of an external magnetic field. As shown in FIG. 13, therefore, dead areas in which reproduction sensitivity is poor are formed in the vicinity of the lateral ends of the multilayer film 33.
Of the multilayer film 33, a central portion except for the opposite dead areas serves as a sensitive area that actually contributes to reproduction of a recorded magnetic field and develops a magnetoresistance effect. A width of the sensitive area is shorter than the track width Tw set in formation of the multilayer film 33 by widths of the opposite dead areas. Also, due to variations in widths of the opposite dead areas, it is difficult to precisely define the track width. This results in a problem of difficulty in narrowing the track width to be adapted for a higher recording density.
In the spin-valve type magnetoresistive sensor shown in FIG. 14, the magnetization direction of the free magnetic layer is uniformly arranged so as to cross the magnetization direction of the pinned magnetic layer at 90° based on exchange biasing by using the bias layers made of an antiferromagnetic material.
The exchange biasing is more suitable for a spin-valve type magnetoresistive sensor having a narrower track width to be adapted for a higher recording density in comparison with hard biasing wherein it is difficult to precisely control an effective track width due to the presence of the dead areas.
However, the spin-valve type magnetoresistive sensor shown in FIG. 14 has a problem of corrosion because the antiferromagnetic layer 22 is formed of a Ni—Mn alloy. Also, in the spin-valve type magnetoresistive sensor including the antiferromagnetic layer 22 formed of a Ni—Mn alloy or a Fe—Mn alloy, another problem is encountered in that the antiferromagnetic layer 22 is corroded by, e.g., a weak alkaline solution, containing natrium tripolyphosphate or the like, and an emulsifier which are used in manufacturing steps of the magnetoresistive head, and hence the exchange anisotropic magnetic field is reduced.
Further, since the antiferromagnetic layer 22 is formed of a Ni—Mn alloy, antiferromagnetic materials usable as the bias layers 26, 26 are restricted. This has necessarily raised such a drawback that the bias layers 26, 26 are poor in heat resistance and corrosion resistance. More specifically, to form the bias layers 26, 26 having high heat resistance, an antiferromagnetic material such as a Ni—Mn alloy must be selected which can develop an exchange anisotropic magnetic field in the direction opposing to the X1-direction at the interface between the bias layers 26, 26 and the free magnetic layer 25 when heat treatment is carried out under a magnetic field crossing the exchange anisotropic magnetic field that acts in the Y-direction in FIG. 14 at the interface between the antiferromagnetic layer 22 formed of an Ni—Mn alloy and the pinned magnetic layer 23.
During the heat treatment under the above-mentioned magnetic field, however, the exchange anisotropic magnetic field acting at the interface between the antiferromagnetic layer 22 and the pinned magnetic layer 23 is inclined from the Y-direction toward the direction opposing to the X1-direction. Accordingly, the magnetization direction of the pinned magnetic layer 23 and the magnetization direction of the free magnetic layer 25 are out of the orthogonal relation, thus resulting in a problem that an output signal waveform has poor symmetry.
For the bias layers 26, 26, it has been therefore required to select an antiferromagnetic material that does not require heat treatment under a magnetic field and can generate an exchange anisotropic magnetic field immediately after the formation under a magnetic field.
For the above reason, the bias layers 26, 26 are generally formed of a FeMn alloy which belongs to the face-centered cubic system and has an irregular crystal structure.
However, when the spin-valve type magnetoresistive sensor shown in FIG. 14 is provided in a magnetic recording device or the like, the sensor is subjected to a high temperature over 100° C. due to a temperature rise in the device and Joule heat produced by a detection electric current. This reduces the exchange anisotropic magnetic field to such an extent that it is difficult to hold the free magnetic layer 25 in a single domain state. As a result, a problem of causing the Barkhausen noise has occurred.
Another problem is that, because the Fe—Mn alloy is poorer in corrosion resistance than the Ni—Mn alloy, the bias layers are corroded by, e.g., a weak alkaline solution, containing sodium tripolyphosphate or the like, and an emulsifier which are used in manufacturing steps of the magnetoresistive head, and hence the exchange anisotropic magnetic field is reduced. In addition, corrosion of the bias layers further proceeds in the magnetic recording device, whereby durability of the device deteriorates.
In the manufacturing method of the conventional spin-valve type magnetoresistive sensor shown in FIGS. 15-17, the surface of the uppermost one of the layers formed between the substrate and the bias layers is exposed to the atmosphere in the step of forming the lift-off resist 351 shown in FIG. 16. The surface having been exposed to the atmosphere must be cleaned by ion milling or reverse sputtering with rare gas, such as Ar, before forming another layer on the exposed surface. This results in a problem of increasing the number of manufacturing steps. Further, the necessity of cleaning the surface of the uppermost layer by ion milling or reverse sputtering raises another problem attributable to the cleaning, such as an adverse effect upon generation of the exchange anisotropic magnetic field caused by contamination with foreign matters deposited again on the surface or disorder of the crystal state at the surface.
In the spin-valve type magnetoresistive sensor MR3 shown in FIG. 18, a strong magnetic field is applied to the first free magnetic layer 177 from tip portions 126a, 126a of the hard bias layers 126, 126 in the vicinity of upper lateral ends of the laminate a12, and this magnetic field is opposed to the direction of a magnetic field to be applied to the first free magnetic layer 177. Therefore, when the magnetic field generated by the hard bias layers 126, 126 becomes greater than a later-described spin flop magnetic field (Hsf), a magnetic field opposing to the direction of the magnetic field, which is intended to be applied to the first free magnetic layer 177, acts upon opposite end portions of the first free magnetic layer 177 (i.e., portions thereof adjacent to the corresponding hard bias layers 126). As a result, the magnetization direction of the first free magnetic layer 177 in its central portion is uniformly arranged in a direction opposing to the magnetization direction of the second free magnetic layer 178 (i.e., in the direction opposing to the X1-direction), whereas the magnetization direction of the first free magnetic layer 177 in its opposite end portions is disordered.
With the magnetization direction disordered in the opposite end portions of the first free magnetic layer 177, the second free magnetic layer 178, of which magnetization direction is to be uniformly arranged (in the X1-direction) in antiparallel relation to the magnetization direction of the first free magnetic layer 177, is brought into such a state that the magnetization direction of the second free magnetic layer 178 in its central portion is uniformly arranged in a direction opposing to the magnetization direction of the first free magnetic layer 177 (i.e., in the X1-direction), whereas the magnetization direction of the second free magnetic layer 178 in its opposite end portions is disordered. Accordingly, the magnetization directions of the first and second free magnetic layers 177, 178 in their opposite end portions are no longer arranged in antiparallel relation. This may accompany a risk of lowering stability of a reproduced waveform at both ends of the track width Tw and hence causing a problem such as a servo error.
A description is now made of the spin flop magnetic field with reference to FIG. 19. FIG. 19 is a graph showing an M-H curve of the free magnetic layer.
The M-H curve represents changes in magnetization M of the free magnetic layer 175 resulted when an external magnetic field H is applied in the direction of the track width to the free magnetic layer 175 of the spin-valve type magnetoresistive sensor MR3 constructed as shown in FIG. 18. Note that, in FIG. 19, the external magnetic field H corresponds to the bias magnetic field from the hard bias layers 126, 126.
Also, in FIG. 19, arrow FL represents the magnetization direction of the first free magnetic layer 177, and arrow F2 represents the magnetization direction of the second free magnetic layer 178.
As shown in FIG. 19, when the external magnetic field H is small, the first free magnetic layer 177 and the second free magnetic layer 178 are in the antiferromagnetically coupled state; namely, the directions of arrows F1 and F2 are antiparallel. When the magnitude of the external magnetic field H exceeds a certain value, the directions of arrows F1 and F2 are not antiparallel and the antiferromagnetic coupling between the first free magnetic layer 177 and the second free magnetic layer 178 is broken, whereby the free magnetic layer 175 can no longer maintain a ferrimagnetic state. This phenomenon is called spin flop transition. Also, the magnitude of the external magnetic field at the time of occurrence of the spin flop transition is called a spin flop magnetic field that is shown by Hsf in FIG. 19. When the external magnetic field H continues to be increased beyond the spin flop magnetic field Hsf, the direction of arrow F1 is further rotated and then becomes parallel to the direction of arrow F2; namely, arrow F1 is pointed in a direction 180° different from the original direction. Thus, the ferrimagnetic state is completely broken. The magnitude of the external magnetic field corresponding to that condition is called a saturation magnetic field that is shown by Hs in FIG. 19.
Accordingly, the magnetization directions of the first and second free magnetic layers 177, 178 in their opposite end portions, shown in FIG. 19, are disordered to a larger extent in the opposite end portions of the first free magnetic layer 177 as indicated, by way of example, by arrows F1 depicted in an area of the first free magnetic layer 177 in FIG. 20. Because of tendency to hold the antiparallel relation in the ferrimagnetic state corresponding to the magnetization direction of the first free magnetic layer 177, the magnetization direction of the second free magnetic layer 178 is also disordered as indicated, by way of example, by arrows F2 depicted in an area of the second free magnetic layer 178 in FIG. 20. In the spin-valve type magnetoresistive sensor MR3 constructed as shown in FIG. 18, therefore, there has been a risk of lowering stability of a reproduced waveform at both ends of the track width Tw and hence causing a problem such as a servo error. To describe in more detail the magnetization state shown in FIG. 20, a strong magnetic field is applied in an opposing direction to the left and right opposite ends of the first free magnetic layer 177 from the hard bias layers 126, 126, whereby magnetization distribution in the second free magnetic layer 178 is also disordered. This invites the occurrence of Barkhausen noise or the like and deteriorates magnetic stability.